The present invention relates to an optical head and an optical information processor using the same, and more particularly to an improved optical head suitable for use in an optical disk apparatus and an optical card reader.
An optical head used in an optical disk apparatus and an optical card reader is required to have a focal error detecting system, since the movement of an information medium in a direction perpendicular to the information recording surface is caused by the rotation or transfer of the information medium. There have been proposed various focal error detecting methods, one of which is the Foucault method. In this method, a light beam emitted from a light source is focused on an information medium by a focusing lens so as to form a fine light spot on the information medium, and the light beam reflected back from the information medium is separated from the light beam which is emitted from the light source, by a beam splitter. The reflected light beam thus obtained is separated by a mirror or the like into two parts, one of which is received by a photodetector separated into a pair of detecting elements. The photodetector is disposed at a position, on which the above light part is focused, and the difference between the outputs of the detecting elements is used as a focal error signal. In the Foucault method, however, a light spot formed on the photodetector has a very small diameter (that is, a diameter of tens of microns) when the light beam incident on the information medium is accurately focused thereon, and thus there arises a problem that a detection error is caused by the slight deviation of an optical element or the like from a predetermined position.
In order to solve the problem, there has been proposed another focal error detecting method. In this method, as described in a Japanese patent application JP-A- No. 54-130,102, the reflected light beam having been separated from the light beam which is emitted from the light source, is separated by a wedge prism into two beams, and two photodetectors each separated into a pair of detecting elements are disposed on a plane, on which the two beams are focused. According to this method, the detection error due to the positional deviation of an optical element can be prevented by adding two focal error signals which are obtained from the photodetectors.
Further, there has been known a different focal error detecting method. In this method, as described in a Japanese patent application JP-A- No. 60-59,545, the light beam reflected from an information medium is separated from the light beam which is emitted from the light source, by a beam splitter, and is then divided into two light beams each having a semicircular cross section. Further, a photodetector separated into two detecting elements is disposed in front of a position, on which one of two light beams is focused, and another photodetector separated into two detecting elements is disposed behind a position, on which the other light beam is focused. A change in size of the semicircular light spot formed on the photodetector due to the movement of an information medium in a direction perpendicular to the information recording surface thereof is used for generating a focal error signal. In more detail, the focal error signal is obtained from the difference between the outputs of two photodetectors each including two detecting elements. When only the output of one photodetector is used as a focal error signal, a change in the focal error signal due to the positional deviation of the information medium in a first direction perpendicular to the information recording surface thereof is not equal to a change in the focal error signal due to the same positional deviation of the information medium in a second direction opposite to the first direction. When the difference between the outputs of two photodetectors is used as a focal error signal, the signal level of the focal error signal is raised or lowered by the same amount, in accordance with whether the information medium is displaced by a desired distance in the first or second direction.
In each of the above methods, when the reflected light beam from the information medium is received by a two-division photodetector which is divided into two photodetector elements along its center line parallel to the longitudinal direction of the recording track on the information medium, a tracking error signal can be obtained from the difference between the outputs of the respective photodetector elements. Further, an information signal indicative of a change in light quantity of the reflected light beam is obtained from the sum of the outputs of two photodetector elements, and an information pit of the perforation type, phase type or phase-changing type can be reproduced from the above information signal.
In a conventional optical head, in which the reflected light beam from an information medium is separated into two parts by a mirror or wedge prism, it has not been taken into consideration to detect the rotation of the plane of polarization of a light beam at a time the light beam is reflected from a magneto-optical disk, thereby reproducing information from the disk. Accordingly, the optical head is required to include optical elements and a photodetector for obtaining a magneto-optical signal, in addition to the optical elements and photodetectors for obtaining the focal error signal and the tracking error signal. Thus, the number of optical elements included in the optical head is increased, and there arises a problem that the optical head becomes expensive and is large in size and weight.